The invention has application to the control of fluid flow and particularly the control of fluids containing abrasive material which can cause blockages and undesirable wear and erosion of control valve parts.
The use of abrasive materials in a fluid jet is well known, for example for machining operations such as cutting, drilling and surface finishing. In one known arrangement, a high pressure carrier fluid, for example water, is pumped into a vessel containing a slurry of abrasive particles to force a mixture of the carrier fluid and abrasive particles through a nozzle which forms a well-defined abrasive jet.
Various applications require the jet, or at least the abrasive content in the jet, to be repeatedly switched on and off. For example, in the sequential drilling of multiple holes the jet, or at least the abrasive content in the jet, is switched off during repositioning of the nozzle over the work piece and then switched on again to drill the next hole.
It is not practical to achieve this by simply repeatedly switching the high pressure pump on and off. The time taken for the alternating depressurisation and repressurisation of the abrasive slurry vessel delays the cessation and re-establishment of the abrasive flow at the jet and limits the jet switching frequency.
Also, when the fluid flow is halted the abrasive particles, which are suspended in the moving flow, can settle and cause blockages, for example in the flow conduits and nozzle. These blockages prevent the easy re-establishment of the abrasive fluid flow and can cause excessive down time while parts are de-pressurised, dissassembled, flushed to clear the blockages, re-assembled and re-pressurised. When the outlet nozzle is blocked, the pressure vessel through which the flow is passed to entrain the abrasive particles must be depressurised through an alternative route, usually a valve connected to the pressure vessel. This valve is subject to a high degree of wear because it is exposed to the full pressure differential across its seat as it opens and because small fines of abrasive from the vessel can pass through the valve.
Furthermore, the use of valves to control the abrasive flow is difficult due to the abrasive nature of the fluid which causes excessive valve wear. Present valves are not entirely suitable for opening and closing against a high-pressure abrasive flow, or for maintaining a pressure differential across the valve during the opening and closing operations.
One prior art two way wiping action valve operates with slurries at low pressures for turning a ‘mostly’ abrasive-free water jet on and off. This type of valve is self-lapping so that as it wears it continues to maintain a seal. However, when controlling high pressure abrasive flows, the valve will quickly become damaged as it nears closing because the high pressure abrasive slurry discharging through the reducing gap will erode the valve.
This latter problem is addressed in the prior art WO 99/14015 and WO 02/087827 which describe the use of small diameter flow outlets to cope with the large pressure differentials, and a controlled jet pump to ensure the flow is relatively abrasive free before actuating the valve. This valve design relies on halting the abrasive flow then shutting the jet, and flow, off completely. For abrasive suspension jets pressurised by piston pumps, stopping and starting the flow is a problem, as is the resultant clogging of the abrasive material in the conduits or hoses if the flow is halted. Furthermore, the small diameter valve outlets make this arrangement unsuitable for larger flow requirements.